This invention relates to carbon fibers and more particularly to continuous carbon fiber yarn having improved handling characteristics including increased abrasion resistance, together with reduction in yarn breakage and low fuzz generation during processing, and to a method for improving the handling characteristics of carbon fiber yarn. While continuous carbon fiber yarn from a variety of sources may be improved when produced according to the invention, high modulus carbon fiber yams including those comprising pitch-based carbon fibers are particularly benefited.
Carbon fibers have long been known, and methods for their production from a variety of precursors are well-described in the art. Cellulosic precursors have been used for producing carbon fiber since the early 1960's, with rayon being the dominant carbon fiber precursor for nearly two decades. More recently, as the art has developed methods for producing carbon fiber derived from such materials as polyacrylonitrile (PAN) and pitch, the importance of rayon-based carbon fiber has declined. This shift has been due in part to the superior toughness, tensile strength and stiffness exhibited by both PAN-based and pitch-based carbon fiber.
Polyacrylonitrile fiber, when oxidized and carbonized under appropriate conditions, provides tough, high strength, high modulus carbon fiber. The overall conversion yield in producing fiber from PAN is good, and the finished fiber is capable of achieving the outstanding tensile strength needed for producing the high performance composite materials used in a variety of sports, automotive and aircraft applications. However, the tensile modulus of commercially available PAN-based fiber does not generally exceed about 50.times.10.sup.6 psi, which is somewhat deficient for use in applications that require a high degree of stiffness.
Pitch-based carbon fiber has generally been recognized as capable of providing greater stiffness than carbon fiber from other sources, and considerable effort has been directed toward the development of pitch-based ultra-high modulus carbon fibers.
Such carbon fibers find application in forming composites for use where good dissipation of electrical charges or heat is desired. In addition, the combination of high stiffness and good thermal conductivity with the negative coefficient of thermal expansion characteristically exhibited by pitch-based fibers provides composites that are extraordinarily dimensionally stable.
Carbon fibers are widely used in the manufacture of aircraft parts, space devices, precision machines, transport devices, sporting goods, and the like because of their excellent mechanical properties, such as specific strength, specific modulus, and chemical resistance. In such uses the carbon fiber is ordinarily used as reinforcement in composite materials comprising a matrix component such as a metal, graphitic carbon, a ceramic, a synthetic resin or the like. Carbon fiber-reinforced composites having synthetic resins as a matrix have found wide acceptance in view of the versatility, uniformity in performance and cost.
Fabricating composites is generally accomplished by processes such as filament winding, and by layup and impregnation using tape and fabric woven from carbon fiber yams. These processes generally require contacting the yam with rollers, guides, spreaders and the like. Continuous carbon fiber, particularly including those carbon fibers having high tensile modulus values designated in the art as "ultra-high modulus", characteristically exhibit poor abrasion resistance and can be regarded as brittle and difficult to handle. The conventional processing thus tends to damage the carbon filaments, which in turn may cause undesirable fuzz, flaws in the resulting composites and loss in production due to yam breakage. A variety of sizes and filer coatings have been devised for use with carbon fiber to improve abrasion resistance and reduce losses.
U.S. Pat. No. 3,837,904 discloses coating the carbon fiber with from 5 to 60 volume per cent of a resin formulation as a size to improve bonding between the matrix and the fiber surface. The coating is generally accomplished after fast surface treating the fiber to etch or pit the surface. The fiber is said to exhibit improved abrasion resistance and reduce fuzzing. Additional sizes are known in the art for this purpose, including, for example, those described in U.S. Pat. No. 4,219,457 and U.S. Pat. No. 4,496,671. Although improvement in handling characteristics is realized for these formulations, as the art has turned to use of higher modulus carbon fiber yam and to more complex and more demanding forming processes and equipment, the prior art sizes have been found inadequate. Further, the use of woven structures for composite fabrication has found greater acceptance in the art. The processes and equipment used in weaving complex structures place great stress on yam and tow surfaces and significantly increases the possibility of damage. In addition, bonding between the fiber and the matrix component that will subsequently be impregnated into the structure is greatly affected by the presence of the size, and it is often desirable to remove the size after weaving, ordinarily in a thermal operation. Prior art sizes, particularly those that are cross-linked, may be difficult to cleanly remove without causing fiber damage.
Methods and sizing formulations for carbon fiber that provide improved yam abrasion resistance and handling character, particularly for high modulus continuous carbon fiber yarn, and that are readily removed after the fabrication operations would thus be a useful advance in the composite manufacturing art.